The nature of the barrier to the utilization of large peptides by E. coli is being investigated by determining the fine structure of the periplasm. BIBLIOGRAPHIC REFERENCES: Barak, Z. and Gilvarg, C. Specialized Peptide Transport System in E. coli (1975) J. Bact. 122. 1200-1207. Gilvarg, C., Jockusch, H. and Weissmann, C., Evidence for the Participation of a Host Enzyme in the Activation of Poly) A)-Q Beta RNA as an Infectious Agent (1975) Biochim. Biophys. Acta 414, 341-348.